Apoptosis is now recognized as an essential biological process for tissue homeostasis of all living species. In mammals in particular, it has been shown to regulate early embryonic development. Later in life, cell death is a default mechanism by which potentially dangerous cells (e.g., cells carrying cancerous defects) are removed. Several apoptotic pathways have been uncovered, and one of the most important involves the Bcl-2 family of proteins, which are key regulators of the mitochondrial (also called “intrinsic”) pathway of apoptosis. See, Danial, N. N. and Korsmeyer, S. J. Cell (2004) 116, 205-219. The structural homology domains BH1, BH2, BH3 and BH4 are characteristic of this family of proteins. The Bcl-2 family of proteins can be further classified into three subfamilies depending on how many of the homology domains each protein contains and on its biological activity (i.e., whether it has pro- or anti-apoptotic function).
The first subgroup contains proteins having all 4 homology domains, i.e., BH1, BH2, BH3 and BH4. Their general effect is anti-apoptotic, that is to preserve a cell from starting a cell death process. Proteins such as, for example, Bcl-2, Bcl-w, Bcl-xL, Mcl-1 and Bfl-1/A1 are members of this first subgroup. Proteins belonging to the second subgroup contain the three homology domains BH1, BH2 and BH3, and have a pro-apoptotic effect. The two main representative proteins of this second subgroup are Bax and Bak. Finally, the third subgroup is composed of proteins containing only the BH3 domain and members of this subgroup are usually referred to as “BH3-only proteins.” Their biological effect on the cell is pro-apoptotic. Bim, Bid, Bad, Bik, Noxa, Hrk, Bmf, and Puma are examples of this third subfamily of proteins. The exact mechanism by which the Bcl-2 family proteins regulate cell death is still not entirely known and understanding this mechanism is an active area of research in the science community. In one hypothesis of regulation of cell death by Bcl-2 family proteins, the BH3-only proteins are further categorized as either “activator” (e.g., Bim and Bid) or “sensitizer” (e.g., Bad, Bik, Noxa, Hrk, Bmf, and Puma) proteins depending on their regulatory function.
The key to tissue homeostasis is achieving the delicate balance in the interactions among the three subgroups of protein in cells. Recent studies have tried to elucidate the mechanisms by which pro-apoptotic and anti-apoptotic subgroups of Bcl-2 family proteins interact to allow a cell to undergo programmed cell death. After receiving intra- or extra-cellular signals in cells, post-translational or transcriptional activation of BH3-only proteins occurs. The BH3-only proteins are the primary inducers of an apoptotic cascade that includes, as one step, the activation of the pro-apoptotic proteins Bax and Bak on the mitochondrial membrane in cells. Upon activation of Bax and/or Bak that are either already anchored to the mitochondrial membrane or migrate to this membrane, Bax and/or Bak oligomerize to result in mitochondrial outer membrane permeabilization (MOMP), the release of cytochrome C, and downstream activation of effector caspases, to ultimately result in cell apoptosis. Some researchers hypothesize that certain BH3-only proteins (e.g., Puma, Bim, Bid) are “activators” in that these proteins directly engage pro-apoptotic proteins Bax and Bak to initiate MOMP, while other BH3-only proteins (e.g., Bad, Bik and Noxa) are “sensitizers” and induce Bax and Bak oligomerization indirectly by binding anti-apoptotic proteins (e.g., Bcl-2, Bcl-xL, Bcl-w, Mcl-1) and displacing and “freeing-up” the “activator” BH3-only proteins, which subsequently bind to and activate pro-apoptotic proteins (e.g., Bax, Bak) to induce cell death. Other researchers suggest that anti-apoptotic proteins engage and seqeuester Bax and Bak directly and all BH3-only proteins regulates this interaction by binding to anti-apoptotic proteins (e.g., Bcl-2, Bcl-xL, Bcl-w, Mcl-1) which results in the release Bax and Bak. See, Adams, J. M. and Cory S. Oncogene (2007) 26, 1324-1337; Willis, S. N. et al. Science (2007) 315, 856-859. Although, the exact interactions through which the anti- and pro-apoptotic Bcl-2 family proteins regulate apoptosis remain under debate, there is a large body of scientific evidence to show that compounds which inhibit the binding of BH3-only proteins to anti-apoptotic Bcl-2 family proteins promote apoptosis in cells.
Dysregulated apoptotic pathways have been implicated in the pathology of many significant diseases such as neurodegenerative conditions (up-regulated apoptosis), such as for example, Alzheimer's disease; and proliferative diseases (down-regulated apoptosis) such as for example, cancer, autoimmune diseases and pro-thrombotic conditions.
In one aspect, the implication that down-regulated apoptosis (and more particularly the Bcl-2 family of proteins) is involved in the onset of cancerous malignancy has revealed a novel way of targeting this still elusive disease. Research has shown, for example, the anti-apoptotic proteins, Bcl-2 and Bcl-xL, are over-expressed in many cancer cell types. See, Zhang J. Y., Nature Reviews/Drug Discovery, (2002) 1, 101; Kirkin, V. et al. Biochimica et Biophysica Acta (2004) 1644, 229-249; and Amundson, S. A. et al. Cancer Research (2000) 60, 6101-6110. The effect of this deregulation is the survival of altered cells which would otherwise have undergone apoptosis in normal conditions. The repetition of these defects associated with unregulated proliferation is thought to be the starting point of cancerous evolution. Additionally, research has shown that BH3-only proteins can act as tumor suppressors when expressed in diseased animals.
These findings as well as numerous others have made possible the emergence of new strategies in drug discovery for targeting cancer: If a small molecule that could mimic the effect of BH3-only proteins were able to enter the cell and overcome the anti-apoptotic protein over-expression, then it could be possible to reset the apoptotic process. This strategy can have the advantage that it can alleviate the problem of drug resistance which is usually a consequence of apoptotic deregulation (abnormal survival).
Researchers also have demonstrated that platelets also contain the necessary apoptotic machinery (e.g., Bax, Bak, Bcl-xL, Bcl-2, cytochrome c, caspase-9, caspase-3 and APAF-1) to execute programmed cell death through the intrinsic apoptotic pathway. Although circulating platelet production is a normal physiological process, a number of diseases are caused or exacerbated by excess of, or undesired activation of, platelets. The above suggests that therapeutic agents capable of inhibiting anti-apoptotic proteins in platelets and reducing the number of platelets in mammals maybe useful in treating pro-thrombotic conditions and diseases that are characterized by an excess of, or undesired activation of, platelets.
Abbott Laboratories Inc. has developed a class of small molecule BH3-only protein mimetics, i.e., ABT-737 and ABT-263, that bind strongly to a subset of anti-apoptotic Bcl-2 proteins including Bcl-2, Bcl-w and Bcl-xL, but only weakly to Mcl-1 and A1, and exhibits mechanism-based cytotoxicity. These compounds were tested in animal studies and demonstrated cytotoxic activity in certain xenograft models as single agents, as well as enhanced the effects of a number of chemotherapeutic agents on other xenograft models when used in combination. See, Tse, C. et al. Cancer Res (2008) 68, 3421-3428; and van Delft, M. F. et al. Cancer Cell (2006) 10, 389-399. These in vivo studies suggest the potential utility of inhibitors of anti-apoptotic Bcl-2 family proteins for the treatment of diseases that involve a dysregulated apoptotic pathway.
The natural expression levels of anti-apoptotic Bcl-2 family proteins members vary in different cell types. For example, in young platelets, Bcl-xL protein is highly expressed and plays an important role in regulating cell death (life span) of platelets. Also, in certain cancer cell types, the cancer cell's survival is attributed to the dysregulation of the apoptotic pathway caused by the over-expression of one or more anti-apoptotic Bcl-2 protein family members. In view of the important role for Bcl-2 family of proteins in regulating apoptosis in both cancerous and normal (i.e., non-cancerous) cells, and the recognized inter-cell type variability of Bcl-2 family protein expression, it is advantageous, to have a small molecule inhibitor that selectively targets and preferably binds to one type or a subset of anti-apoptotic Bcl-2 protein(s), for example, to an anti-apoptotic Bcl-2 family member that overexpressed in a certain cancer type. Such a selective compound also may confer certain advantages in the clinical setting, by providing, for example, the flexibility to select a dosing regimen, a reduced on-target toxic effect in normal cells, among others (e.g., lymphopenia has been observed in Bcl-2 deficient mice). See, Nakayama, K. et al. PNAS (1994) 91, 3700-3704.
In view of the above, there is a need in the art for small molecules therapeutics that can selectively inhibit the activity of one type or a subset of anti-apoptotic Bcl-2 proteins, for example, of a Bcl-xL anti-apoptotic protein. The present invention fulfills at least this need.